Great to hear how it went Sharon. As I understood it's a while until the wedding and you will hopefully have some opportunities to talk to both your sister and your son until the final arrangements are made.
It was nice to se that he didn't have any protest against wearing a dress and that he brought this up himself.
-But this raises another dilemma as I see it: In case he's got crossdressing tendencies (and he might) - will it be right to deny him to wear a dress? What might he be feeling if you said that boys must wear tuxedos and he is really starting to "come out" to you?
I would at least let the dress be an option, -maybe let him try on some and find time to talk to him and see if he want to "be a girl".
During the planning you can have a lot of opportunities to show pictures of both dresses, tuxes and other things. It's a great opprotunity to talk normally about this and not make it a problem (and make him think that beeing a boy is a problem..)
oki... I may be rambling away here and it may not be so serious as I said... but what if?
Anyway it's supposed to be a fun time - and I hope you all have a great time planning and preparing the wedding!
And if he want to to wear a dress - Let him do it!
(Don't let other ppl's opinions be the only one that counts)